Structure and Function of the Dimeric and Tetrameric Hemoglobins from the Bivalve Mollusc, Anadara Broughtonii

Abstract

Clams of the primitive family Arcidae (the so-called blood clam) have dimeric and tetrameric hemoglobin components in the nucleated corpuscles (1–5). A few species of the Arcidae contain homodimeric (Hb I) and α2β2 tetrameric forms (Hb II) [Anadara broughtonii (2), Anadara trapezia (4), Scapharca inaequivalvis (5)]. Both hemoglobin components bind oxygen cooperatively, without the alkaline Bohr effect. The tetrameric hemoglobin polymerizes in the deoxygenated state (5,6). Very recently, Royer et al. (7) have determined the crystal structures of dimeric (Hb I) and tetrameric hemoglobins (Hb II) from S. inaequivalvis to a 5.5°A resolution. The subunits of both molecules assemble ‘back to front’, relative to mammalian hemoglobins. Thus, the studies show that certain Arcidae pigments share distinct structural and functional characteristics with each other. We previously reported the amino acid sequence of the homodimeric Hb I from A. broughtonii (8). The sequences of A. trapezia globins (9,10) have also been determined.